Our long-term objective is to understand the biochemical pathway that organizes dorsal-ventral polarity of the Drosophila embryo. This polarity is established shortly after fertilization by maternal gene products. An embryo missing any of these maternal components fails to form the normal dorsal-ventral pattern of anatomical structures, because all cells of the mutant embryo follow the developmental path of either dorsal or ventral cells of the wild type embryo. Embryonic cells are informed of their dorsal-ventral positions by a morphogen gradient of the dorsal protein, which is structurally similar to the product of the vertebrate proto-oncogene c-rel. The dorsal protein gradient is organized in the syncytial embryo by 11 other maternal gene products. Genetic and molecular studies suggest that these molecules function in a transmembrane signaling pathway in which the membrane-bound Toll protein acts as a signal transducing receptor. The Toll protein shares structural features with many cell surface receptors; for example, its cytoplasmic domain is similar to that of the mammalian interleukin 1 receptor, which mediates many cellular activities in the immune response. Because regulated Toll protein activity is critical for organizing the dorsal protein gradient and hence embryonic dorsal-ventral polarity, our specific goal is to understand how the Toll protein functions biochemically and how its activity is spatially regulated in the embryo. Our specific aims are: 1) We will use biochemical and immuno-localization methods to define the Toll protein's structural properties and subcellular distribution relevant to its functioning. 2) We will assay the activities of wild type and-mutant Toll proteins in transplantation experiments with embryos and in cell adhesion using transfected cultured cells, with the aim of reconciling how this protein could appear to function as both signal transducing receptor and cell adhesion molecule. 3) We plan to identify biochemically molecules that directly interact with the Toll protein, since characterization of these molecules will provide clues to Toll's biochemical function and how its activity is spatially regulated. These experiments should also expand our knowledge of the molecules involved in the biochemical pathway that organizes dorsal-ventral polarity of the Drosophila embryo.